On the brink of a painful government shutdown, the Obama administration readied furlough notices for hundreds of thousands of workers Friday as Republican and Democratic leaders accused each other of refusing to give ground on a deal to keep operations running.

By midday Friday, most employees of the federal workforce had been told whether they had been deemed essential or would be temporarily laid off from work if lawmakers failed to reach an agreement by midnight. In the event of a shutdown, official furlough notices would begin going out by email, by written letter or in person.

Many workers would be allowed into their offices for up to four hours on Monday to finish tasks, but that would be it.

Senate Majority Leader Harry Reid accused the Republicans of blocking a compromise because they were determined to make it harder "for women to get cancer screenings." That was a reference to money for Planned Parenthood, an organization Republicans assail as the country's largest provider of abortions.

Reid's main antagonist in the long-running negotiations, House Speaker John Boehner, said spending cuts - not social issues - were blocking agreement to prevent a shutdown.

"Most of the policy issues have been dealt with, and the big fight is about spending," Boehner said Friday afternoon.

Barring an agreement or perhaps another temporary bill to keep the government operating, the shutdown of most of the government would begin at midnight. Many essential workers, such as mail carriers, air traffic controllers and the military, would stay on the job, but national parks would close and pay for troops and other workers could be delayed.

President Barack Obama canceled a trip to Indianapolis Friday to stay in Washington and spoke in separate phone calls with Reid and Boehner Friday morning.

But with no compromise in sight, the White House found itself in a holding pattern as the shutdown clock ticked toward midnight. Aides said there were no plans for the president to summon congressional leaders to the White House.

In another sign of growing uncertainty over an agreement, Obama canceled a weekend trip with his family to Colonial Williamsburg in Virginia.

Obama, Reid and Boehner met at the White House Thursday night for the third time in two days, and their aides struggled without success to reach agreement in middle-of-the-night talks in the Capitol.

Republicans have been seeking $40 billion in cuts, as well as several other provisions to advance the conservative agenda backed by a rank and file that includes dozens of first-termers elected with the support of tea party activists.

Reid said the two sides had reached agreement on $38 billion in spending cuts and the only hang-up was a Republican demand to cut a federal program that provides women's health and family planning services.

"They are willing to throw women under the bus, even if it means they'll shut down the government," he said. "Republican leaders in the House have only a few hours left to look in the mirror, snap out of it and realize how positively shameful that would be."

But Boehner said there was "only one reason that we do not have an agreement as yet, and that issue is spending."

"When will the White House and when will Senate Democrats get serious about cutting federal spending?" he asked.

Boehner urged Obama to reconsider a veto threat against legislation to keep the government open for one additional week while negotiators continue working on a deal to fund federal programs through Sept. 30.

The short-term measure includes $12 billion in spending cuts and would provide enough funds to keep the Pentagon in operation through Sept. 30.

Obama said ominously on Friday night that the machinery of a shutdown was already in motion.

Congressional aides were trying to cobble together a deal on how much federal spending to slash, where to cut it and what caveats to attach as part of a bill to fund the government through the end of the budget year on Sept. 30. The most recent temporary federal spending measure expires at midnight.

For a nation eager to trim to federal spending but also weary of Washington bickering, the spending showdown had real implications.

A closure would mean the furloughs of hundreds of thousands of workers and the services they provide, from processing many tax refunds to approving business loans. Medical research would be disrupted, national parks would close and most travel visa and passport services would stop, among many others.

Republicans want deeper spending cuts than the Democrats favor and also are pressing for provisions to cut off federal funds to Planned Parenthood and to stop the Environmental Protection Agency from issuing numerous anti-pollution regulations.

"They're difficult issues. They're important to both sides and so I'm not yet prepared to express wild optimism," the president said.

There was agreement that a shutdown posed risks to an economy still recovering from the worst recession in decades.

The political fallout was less predictable, especially with control of government divided and dozens of new tea party-backed Republicans part of a new GOP majority in the House. Twin government shutdowns in the mid-1990s damaged Republicans, then new to power in Congress, and helped President Bill Clinton win re-election in 1996.

This time, individual lawmakers worked to insulate themselves from any political damage. Sens. Sherrod Brown, D-Ohio, and Ben Nelson, D-Neb., both seeking new terms in 2012, became the latest to announce they would not accept congressional pay during any shutdown. "If retroactive pay is later approved, I'll direct my part to the U.S. Treasury," Nelson said. Some two dozen senators of both parties scurried to make similar pledges.

There also were hints Thursday of Republican flexibility on the ban they were seeking to deny federal funds to Planned Parenthood. Officials said Republicans had suggested giving state officials discretion in deciding how to distribute family planning funds that now go directly from the federal government to organizations such as Planned Parenthood.

That would presumably leave a decision on funding to governors, many of whom oppose abortion, and sever the financial link with the federal government.